DNA repair systems are important defenses in the prevention of cancer. Breakdowns in these systems can lead to a variety of negative physiological effects, not least of which is cancer. The nucleotide excision repair system is one of the cell's key defense mechanisms against DNA damage, recognizing the bulky DNA lesions produced by UV sunlight and carcinogens derived from smoking. It operates by recognizing a damaged nucleobase and excising a DNA fragment of 24-32 nucleotides containing the damaged nucleotide, then synthesizing new DNA to replace the excised DNA. The proteins involved in this system are XPC-HR23B, XPA, RPA, TFIIH, XPG, XPF-ERCC1, RFC, PCNA, Pol epsilon or delta and DNA Ligase I. XPC-HR23B, XPA, RPA, TFIIH, XPG and XPF are involved in recognizing and excising the damaged DNA. RPA, TFIIH, XPG and XPF-ERCC1 all have other cellular functions besides their role in nucleotide excision repair, XPC-HR23B and XPA, on the other hand, are known to only function in excision repair. Interestingly XPC-HR23B and XPA, along with RPA, are the proteins responsible for initial DNA damage recognition. XPA and RPA have been well characterized biochemically, however less is known about how XPC-HR23B functions. The goal of this proposal will be to gain a greater understanding of how XPCHR23B interacts with damaged DNA. This will be accomplished by mapping out the DNA binding domain of XPC-HR23B and determining the detailed binding kinetics of XPC-HR23B to damaged DNA. These studies should give insight into how just three proteins, XPA, RPA and XPC-HR23B can recognize literally hundreds of different DNA lesions and may lead to new treatments for cancer.